Impact of Three Ampicillin Dosage Regimens on Selection of Ampicillin Resistance in Enterobacteriaceae and Excretion of bla TEM Genes in Swine Feces

Abstract

ABSTRACT The aim of this study was to assess the impact of three ampicillin dosage regimens on ampicillin resistance among Enterobacteriaceae recovered from swine feces by use of phenotypic and genotypic approaches. Phenotypically, ampicillin resistance was determined from the percentage of resistant Enterobacteriaceae and MICs of Escherichia coli isolates. The pool of ampicillin resistance genes was also monitored by quantification of bla TEM genes, which code for the most frequently produced β-lactamases in gram-negative bacteria, using a newly developed real-time PCR assay. Ampicillin was administered intramuscularly and orally to fed or fasted pigs for 7 days at 20 mg/kg of body weight. The average percentage of resistant Enterobacteriaceae before treatment was between 2.5% and 12%, and bla TEM gene quantities were below 10 7 copies/g of feces. By days 4 and 7, the percentage of resistant Enterobacteriaceae exceeded 50% in all treated groups, with some highly resistant strains (MIC of >256 μg/ml). In the control group, bla TEM gene quantities fluctuated between 10 4 and 10 6 copies/g of feces, whereas they fluctuated between 10 6 to 10 8 and 10 7 to 10 9 copies/g of feces for the intramuscular and oral routes, respectively. Whereas phenotypic evaluations did not discriminate among the three ampicillin dosage regimens, bla TEM gene quantification was able to differentiate between the effects of two routes of ampicillin administration. Our results suggest that fecal bla TEM gene quantification provides a sensitive tool to evaluate the impact of ampicillin administration on the selection of ampicillin resistance in the digestive microflora and its dissemination in the environment.